This invention pertains to magnetic recording media such as magnetic disks.
FIG. 1 illustrates a magnetic recording medium 1 constructed in accordance with the prior art used for longitudinal data recording. Medium 1 comprises a substrate 2, an underlayer 3, a Co alloy magnetic layer 4 and a carbon protective overcoat 5. Also shown in FIG. 1 is a first region 4a of layer 4 magnetized in a first direction D1, a second region 4b magnetized in a second direction D2 opposite the first direction, and a transition region TR between regions D1 and D2. In magnetic recording, it is desirable for transition region TR to be as small as possible in order to maximize areal recording density. In general, the length of transition region TR is proportional to MrT/Hc, where Mr is the magnetic remanence of the Co alloy, T is the thickness of layer 4, and Hc is the coercivity of layer 4.
In order to reduce the length of region TR, one might be tempted to reduce MrT. Unfortunately, reducing MrT in medium 1 reduces the thermal stability of layer 4. In other words, reducing MrT reduces the ability of layer 4 to retain its magnetization state, and hence the data recorded in layer 4, as temperature increases. (Obviously, thermal stability is a highly desirable characteristic in a magnetic medium.)
FIG. 2 illustrates a second magnetic recording medium 20 in accordance with the prior art comprising a substrate 22, an underlayer 24, a lower Co alloy magnetic layer 26, a Ru layer 28, an upper Co alloy magnetic layer 30 and a carbon protective overcoat 32. Medium 20 is designed to facilitate simultaneous reduction of the length of transition region TR and increase in thermal stability. In particular, if Ru layer 28 has a thickness within a certain range (e.g. 0.3 to 1.0 nm), magnetic layers 26 and 30 are antiferromagnetically coupled to one another. Because of this, the length of transition region TR for medium 20 is proportional to Mr26T26−KMr30T30, where K is a proportionality constant, Mr26 is the magnetic remanence of layer 26, T26 is the thickness of layer 26, Mr30 is the magnetic remanence of layer 30 and T30 is the thickness of layer 30. (Constant K is related to the degree of antiferromagnetic coupling between layers 26 and 30.) However, the thermal stability of medium 20 increases as a function of Mr26T26+K2Mr30T30. Thus, while the antiferromagnetic coupling permits one to reduce the length of transition region TR, it also improves thermal stability. (The antiferromagnetic coupling also improves the signal to noise ratio of medium 20.)
When recording data in medium 20 of FIG. 2, because of the antiferromagnetic coupling between layers 26 and 30, when one magnetizes a region within layer 30, e.g. as shown by arrow D3, the magnetization direction of layer 26 is in the opposite direction, e.g. as shown by arrow D4. FIG. 3 shows a hysteresis loop 40 for the structure of FIG. 2 if layers 26 and 30 were strongly coupled. As can be seen, as one increases the applied magnetic field Happ to medium 20, in portion P1 of hysteresis loop 40, both magnetic layers 26 and 30 are magnetized in the same direction D3. As one reduces the applied magnetic field Happ past point P2, the magnetization direction of layer 26 begins to switch to direction D4. Portion P3 of hysteresis loop 40 shows the magnetic characteristics of medium 20 as layer 26 changes magnetization direction in response to applied magnetic field Happ. As applied magnetic field Happ is brought to zero (point P4), layer 26 is magnetized in direction D4.
FIG. 4 shows a hysteresis loop of medium 20 if layer 26 were weakly antiferromagnetically coupled to layer 30. As can be seen, the point P5 at which layer 26 switches magnetization direction (i.e. from direction D3 to D4) occurs at a much lower applied magnetic field Happ in FIG. 4 than in FIG. 3. Weak coupling between layers 26 and 30 is disadvantageous because it increases the amount of time required to switch the state of medium 20 to an antiferromagnetic state. In particular, it takes more time to create a situation in which layer 30 is magnetized in direction D3 and layer 26 is magnetized in direction D4. The magnetic recording medium relies on thermal energy to switch the magnetization direction of layer 26 to direction D4. Further, weak coupling can also create a situation in which layer 26 is not as completely magnetized as desired when Happ is brought to zero.
It would be desirable to increase antiferromagnetic coupling between layers 26 and 30. One way to do this is to add pure Co layers 34 and 36 on each side of Ru layer 28, e.g. as provided in medium 20′ shown in FIG. 5. Co layers 34 and 36 increase antiferromagnetic coupling between layers 26 and 30. Unfortunately, Co layers 34 and 36 increase noise in medium 20′ because of intergranular magnetic coupling in layers 34 and 36. It would be highly desirable to increase antiferromagnetic coupling between layers 26 and 30 without suffering this increase in noise.